During transportation, trailers are often loaded onto rail cars. A mobile crane 1 for loading such a trailer 2 is shown in FIG. 1. Such mobile cranes 1 typically having a lift carriage 4 containing four arms 5. When not in use, the arms 5 are rotated upward into a storage position about pivot joints 28, as shown in phantom in FIG. 2. When the lift carriage 4 is in position over the trailer 2, the arms 5 are rotated downward and the lower portion of each arm is extended along the sides of the trailer so that a lifting foot 6, attached to the distal end of each arm, can engage the underside of the trailer, as shown in FIGS. 2 and 3.
As shown in FIG. 3, each lifting foot 6 is comprised of front and back plates 9 and 10, respectively, to which a pad 11 is attached. In use, the pad 11 extends in a horizontal plane to allow it to contact the underside of the trailer 2. A cushion 12 is attached to the active surface of the pad 11. The lifting foot 6 is rotatably coupled to the distal end of the arm 5 by a clevis pin 13. The clevis pin 13 allows the foot 6 to automatically swivel into the proper orientation--that is, with the pad 11 extending in a horizontal plane--for engaging the underside of the trailer 2 when the arms 5 are lowered.
In the past, lifting feet were designed to load trailers approved by the Association of American Railroads (AAR). Such trailers have a support beam 7, shown in FIG. 1, formed on their bottom to distribute the weight of the trailer when held by the feet 6. In addition, some AAR trailers, such as trailer 3 shown in FIG. 2, have a drop frame 8 extending downward from their underside which limits the space available for engagement of the foot. Consequently, the length of the pad 11 has traditionally been limited to 30 inches, the maximum length foot which is capable of engaging a drop frame trailer 3.
The aforementioned limitation in the length of the lifting foot pad 11 presented no structural problem in AAR approved trailers due to the presence of the support beam 7, as previously discussed. However, highway trailers, which need not obtain AAR approval and, typically, do not incorporate the support beam 7, can also be transported on rail cars subject to certain limitations. Unfortunately, in the absence of the support beam 7, the 30 inch long pad 11 of a traditional lifting foot 6 is not long enough to adequately distribute the weight of the trailer, thereby creating a risk of structural damage to the trailer.
Accordingly, it would be desirable to develop a lifting foot which has a pad small enough to engage AAR approved trailers with drop frames yet which could readily be made large enough to support highway trailers.